This invention relates to a polarization control system for use in an optical communication system which is typically either a bidirectional coherent optical communication system or a frequency division multiplexed (FDM) optical signal distributing system.
Having diligently continued reduction to practice of this invention after filing in Japan, the present applicants contributed, together with two others, a letter to the IEEE Photonics Technology Letters. The letter was printed in Volume 2, No. 2 (Feb. 1990), pages 135 to 138, under the title of "A Bidirectional Common Polarization Control Method for Coherent Optical FDM Transmission System". The printed letter is herein incorporated by reference.
In general, an optical communication system comprises an optical fiber having first and second ends. The optical fiber is, for example, from 20 km to 100 km long and has a birefringence which is inevitably subjected at various parts of the length to variations by the ambient temperature of the optical fiber, external pressure supplied to the optical fiber, or both. It is therefore unavoidable that a signal beam has a polarization state which undergoes fluctuations while transmitted through the optical fiber.
In the optical communication system, a first or first-end transmitter device and a first or first-end receiver device are connected to the first end. A second or second-end transmitter device and a second or second-end receiver device are connected to the second end. The first transmitter device is for generating a first signal beam which is transmitted through the optical fiber from the first end to the second end to be received by the second receiver device. The second transmitter device generates a second signal beam for transmission through the optical fiber from the second end to the first end and for reception by the first receiver device. Due to the fluctuations in the polarization state, each receiver device can not always have a best reception sensitivity.
It is possible to avoid occurrence of the fluctuations and to always achieve the best receiver sensitivity if the optical fiber is of a polarization maintaining type. A polarization maintaining optical fiber is, however, very expensive. It is therefore usual to use a single-mode optical fiber and a polarization control system in order to suppress the fluctuations.
The polarization control system may be operable according either to automatic polarization control or to polarization diversity. When the polarization diversity is resorted to, each receiver device must have a complicated structure. Moreover, the reception sensitivity becomes poor. It is therefore preferred to use the automatic polarization control for simplification of each receiver device and for achievement of a high reception sensitivity.
In a conventional polarization control system, first and second polarization control devices are used near the first and the second ends to suppress the fluctuations in the polarization state of the second signal beam arriving at the first receiver device and the fluctuations in the polarization state of the first signal beam reaching the second receiver device. This undesiredly raises the price of the polarization control system and consequently the price of the optical communication system.